A new laboratory study, published in MicrobiologyOpen, suggests that a solution may lie in one of the most abundant molecules inside every living cell: ribosomal RNA. Previous work on prokaryotes demonstrated that when viruses lyse bacterial cells, substantial amounts of rRNA are released into the extracellular medium, where it can be detected and quantified. Building on that observation, researchers developed an approach called Mortality by Ribosomal Sequencing, or MoRS, which compares ribosomal RNA sequences in cellular and dissolved fractions of seawater to estimate taxon-specific cell lysis. What remained untested was whether the concept could be extended to eukaryotic microalgae, whose complex internal organization and diverse viral enemies make such an extrapolation far from trivial.

To test the idea, a research team led by investigators working with marine algal virus systems conducted controlled infection experiments with two contrasting host-virus pairs. The first was the diatom Chaetoceros tenuissimus, a small coastal phytoplankton species less than ten micrometers across that is susceptible to Chaetoceros tenuissimus RNA virus type II, a small single-stranded RNA virus with particles roughly 22 to 38 nanometers in diameter. The second was the raphidophyte Heterosigma akashiwo, a globally distributed, flagellated species notorious for forming harmful algal blooms, which was challenged with HaV strain 120, a large double-stranded DNA virus with strain-specific infectivity. Together, these systems span different host physiologies and viral genome types, providing a rigorous framework for testing whether cell-free rRNA behaves as a general lysis marker in eukaryotes.

The experimental design was carefully controlled. Host cultures were grown at 22 degrees Celsius under a 12-hour light and 12-hour dark cycle in sterile artificial seawater media and acclimated for multiple generations before infection. For each host-virus system, triplicate flasks received virus at a multiplicity of infection of ten, while triplicate control flasks received none. The cultures were incubated for six days and sampled daily. To track viral proliferation, the team used the most probable number method to determine infectious virus titers at the beginning and end of the incubation. Critically, they also added purified Escherichia coli ribosomes to every flask as spike-in controls, at a concentration of approximately 2.9 times ten to the seventh copies per milliliter, allowing them to measure how quickly free rRNA degrades in the medium.

Separating the cellular from the dissolved fraction was achieved by gentle filtration through 0.22-micrometer polycarbonate filters under low vacuum. RNA retained on the filter was operationally defined as the cellular fraction, while RNA passing through was defined as cell-free. Cellular RNA was extracted with a silica-column kit enhanced by bead-beating, whereas cell-free RNA was concentrated from twenty-milliliter filtrates using a column-based vacuum system optimized for large water volumes. After genomic DNA removal and complementary DNA synthesis, absolute quantification was performed with a digital PCR system, using newly designed primers targeting the V4 region of the algal 18S rRNA gene and primers specific to the major capsid protein genes of each virus. Extraction efficiencies of 21.9 percent for the cellular fraction and 83.1 percent for the cell-free fraction were used to correct the measured concentrations.

The growth dynamics of the two hosts diverged sharply. In the Chaetoceros experiment, cells in the virus-added treatment grew at rates comparable to controls until day three but then grew significantly more slowly, while cell density, chlorophyll a, and cellular rRNA all continued to rise until day three or four. In striking contrast, Heterosigma cultures declined immediately after viral addition, with most cells losing motility within 24 hours, echoing earlier reports on this host-virus system. Infectious virus particles increased dramatically in both systems, from 5.8 times ten to the fifth to 1.9 times ten to the eighth most probable number units per milliliter for the diatom RNA virus, and from 7.6 times ten to the fifth to 1.4 times ten to the seventh for the Heterosigma DNA virus, confirming active viral replication.

The central result concerned the redistribution of rRNA between particulate and dissolved pools. In the Chaetoceros experiment, cell-free 18S rRNA remained low in controls, below about 4 times ten to the eighth copies per milliliter, but rose sharply in infected flasks to a maximum of 3.7 times ten to the ninth copies per milliliter on day four. Per-cell rRNA content varied up to 2.8-fold within each condition but showed no significant treatment effect, and cellular rRNA correlated strongly with cell density in both species, with Pearson correlation coefficients of 0.72 and 0.94. These findings established that rRNA abundance tracks plankton biomass at the population scale, while its release into the dissolved phase signals cell rupture.

To convert rRNA dynamics into lysis rates, the team built a rate-based model describing daily changes in cell-free rRNA concentration. The model rests on first-order degradation kinetics, an assumption validated by the spike-in ribosomes, which decayed exponentially with rate constants of roughly 1.78 to 2.16 per day in the Chaetoceros experiment, corresponding to half-lives of 7.7 to 9.3 hours. By rearranging a simple mass-balance equation, the researchers solved for the cell-free rRNA production rate, a direct proxy for cell lysis. Normalizing this production rate by host cell abundance yielded a metric they call cell-free rRNA production per cell. In the Chaetoceros system, this metric was 46.1-fold higher in infected treatments than controls between days two and three, and in the Heterosigma system it reached 302.1-fold higher between days three and four, with maximum per-cell release rates up to 46.1- and 302.1-fold above controls across the experiments.

The two host-virus systems also revealed fundamentally different infection strategies. In Chaetoceros, viral marker genes accumulated progressively in the cellular fraction over days one to four, and lysis peaked between days two and three even as the population was still growing, demonstrating that active viral lysis can occur during the apparent growth phase of a bloom. In Heterosigma, elevated per-cell rRNA release appeared both early, during days zero to one, and late, during days three to five. The early mortality could not be explained by canonical viral lysis, since viral capsid gene expression peaked only on day two. The authors suggest several possible mechanisms, including abortive infection, membrane destabilization during viral entry, a phenomenon analogous to lysis from without described in bacteriophage systems, or damage caused by bacteria introduced with the nonaxenic viral lysate, which flow cytometry confirmed had proliferated to high densities in infected treatments.

The implications reach well beyond the laboratory. Unlike dilution assays, which are labor-intensive and lack taxonomic resolution, cell-free rRNA quantification combined with high-throughput sequencing could simultaneously assess lysis across diverse taxa in natural communities, offering a taxon-resolved measure of lytic mortality that plankton ecologists have long sought. Because viral lysis is estimated to contribute up to roughly 45 percent of the labile dissolved organic carbon pool in marine systems, better quantification of who is being lysed, and when, could sharpen models of carbon cycling and microbial loop dynamics. The authors caution that the approach has limits: ribosome content per cell varies several-fold, infected bacteria can actively reduce their ribosome pools, degradation constants differ among seawater samples and must be measured for each one, and nonviral processes such as sloppy feeding, parasitism, and algicidal bacteria can also release intracellular rRNA. They also note that the degradation kinetics of E. coli 16S rRNA may not perfectly match those of eukaryotic 18S rRNA, calling for spike-in standards derived from representative plankton. Even with these caveats, the study provides a mechanistic foundation for reading cell death directly from the RNA dissolved in seawater, turning a ubiquitous molecule into a quantitative witness of the viral shunt at work.

Subject of Research: Quantifying viral lysis of eukaryotic microalgae using cell-free ribosomal RNA as a molecular marker

Article Title: Quantifying Viral Lysis in Microalgae Using Cell‐Free rRNA

Article References: Kikuya, S., Tomaru, Y., Nagasaki, K., Morimoto, D., Yamagishi, Y., Ogata, H., & Endo, H. (2026). Quantifying Viral Lysis in Microalgae Using Cell‐Free rRNA. MicrobiologyOpen, 15(5), Article e70402. https://doi.org/10.1002/mbo3.70402

Image Credits: AI Generated

DOI: 10.1002/mbo3.70402

Keywords: viral lysis, microalgae, ribosomal RNA, phytoplankton, viral shunt, marine viruses, Chaetoceros tenuissimus, Heterosigma akashiwo, MoRS, digital PCR, plankton mortality, biogeochemistry

The question matters because plankton death is not simply an endpoint. When plankton cells die, their decomposing remains release dissolved organic carbon into the surrounding seawater, a form of carbon that microbes can transform and that can be stored in the ocean for thousands of years. The fate of this carbon shapes everything from microbial food webs to the ocean’s long-term capacity to sequester greenhouse gases. Understanding the mechanisms behind plankton mortality is therefore essential for reconstructing how marine ecosystems function and how material flows through them. The trouble is that a single plankton community can consist of hundreds of coexisting species, and pinpointing how many cells within such a crowded assemblage are dying, and at what rate, has long eluded researchers.

A team at Kyoto University has now tackled this challenge by focusing on one of the most pervasive causes of plankton death: viral infection. Viruses are extraordinarily abundant in seawater, and when they infect a plankton cell they often trigger cell lysis, the process by which the cell’s membrane breaks down and its contents, including genetic material, spill into the environment. This invisible death releases ribosomal RNA, or rRNA, into the water. Rather than trying to count dying cells directly, the researchers reasoned that they could measure the RNA these cells leave behind, turning the genetic debris of viral lysis into a quantitative signal of mortality.

To build such a measurement, the team first grew laboratory cultures of two phytoplankton groups, diatoms and raphidophytes, in a seawater-based medium. They then extracted the rRNA present in the medium and quantified it using digital PCR, a highly sensitive technique capable of counting individual nucleic acid molecules. The approach faced a fundamental obstacle, however: rRNA released into seawater does not persist. It degrades over time, meaning that any measured concentration reflects both the ongoing production of cell-free rRNA and its simultaneous disappearance. Without accounting for degradation, the method would systematically underestimate how much RNA dying cells actually release.

The researchers solved this problem with an elegant trick borrowed from analytical chemistry. They introduced a culture of spike-in ribosomes, a known quantity of ribosomal material that was not produced by the plankton, into the medium and tracked how quickly it degraded. This gave them a degradation rate constant specific to their experimental conditions. They then built a flux model that incorporated both the measured changes in host cell-free rRNA over time and this degradation constant. With both terms in hand, the model could correct for the RNA that had already broken down, making it possible to estimate the true rate of cell lysis at any given moment in the experiment.

The results were striking. Viral infection enhanced the rate of cell-free rRNA production approximately 46-fold compared with uninfected cultures, and subsequent cell lysis boosted it roughly 302-fold. In the non-infected solutions, only very small amounts of rRNA were actively released by living cells, underscoring how strongly lysis signals stand out from the background noise of a healthy population. The method effectively turns viral mortality into a measurable molecular beacon, one that can be detected while the deaths themselves are happening.

Perhaps the most surprising finding emerged from the timing. In the diatom experiment, dissolved rRNA production peaked before the population density began to decline as a result of viral infection. In other words, active cell lysis was already underway while the population as a whole was still growing steadily. From a biogeochemical perspective, this implies that the supply of dissolved organic matter to the environment through cell death may occur primarily during the growth phase of a bloom, rather than during its apparent decline, as conventional observations would suggest. Cells are dying and leaking their contents into the water long before anyone watching the population curve would notice.

We did not expect the temporal decoupling between population declines and cell lysis, says corresponding author Hisashi Endo of Kyoto University. Observing the dynamics of living cells is not enough to evaluate the dissolved organic carbon that phytoplankton contribute to marine environments. The statement carries significant weight for oceanographers who model carbon cycling, because it suggests that mortality-driven carbon fluxes may be systematically misattributed in time if they are inferred solely from changes in cell abundance. Carbon could be flowing into microbial food webs and the deep ocean at moments when blooms appear to be thriving.

The methods developed by the Kyoto team for quantifying this invisible death of plankton offer a valuable new lens for understanding material flows within ecosystems. The researchers are careful to note the study’s limits. The reasons for plankton mortality are diverse, spanning grazing, nutrient starvation, disease and viral attack, and this study did not distinguish between different causes of cell lysis. The flux model detects death, but not its perpetrator. The team now intends to develop approaches that can track both the impacts and the causes of cell lysis at the species level, a step that would allow ecologists to attribute carbon release to specific pathogens and specific hosts within complex natural communities.

For Endo, the work is the continuation of a long-standing fascination with the viral dark matter of the sea. Since we revealed that a wide variety of viruses are present in seawater, I have been interested in understanding their impact on the ecosystem, he says. Using this research as a starting point, I hope to shed light on the true nature of the plankton ecosystem. As the technique matures and moves from laboratory cultures toward field applications, it could transform how scientists monitor ocean health, refine global carbon models, and appreciate the ceaseless, mostly invisible cycle of life and death playing out in every drop of seawater.

Subject of Research: Quantifying viral cell lysis of marine plankton using extracellular ribosomal RNA

Article Title: Dead or alive, plankton support marine ecosystems

Article References: Dead or alive, plankton support marine ecosystems. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: plankton, viral lysis, phytoplankton, rRNA, digital PCR, dissolved organic carbon, marine ecosystems, biogeochemical cycles, diatoms, raphidophytes, carbon sequestration, ocean viruses